1. Field of the Invention
The present invention relates to the field of high density interconnect structures for interconnecting electronic components, and more particularly, to such structures intended for high frequency operation.
2. Background Information
A high density interconnect (HDI) structure or system which has been developed by General Electric Company offers many advantages in the compact assembly of electronic systems. For example, an electronic system such as a micro computer which incorporates 30-50 chips can be fully assembled and interconnected on a single substrate which is 2 inch long by 2 inch wide by 0.050 inch thick. Even more important, this interconnect structure can be disassembled for repair or replacement of a faulty component and then reassembled without significant risk to the good components incorporated within the system. This repairability is a substantial advance over prior connection systems in which reworking the system to replace damaged components was either impossible or involved substantial risk to the good components.
Briefly, in this high density interconnect structure, a ceramic substrate such as alumina which may be 100 mils thick and of appropriate size and strength for the overall system, is provided. This size is typically less than 2 inches square, but may be made larger or smaller. Once the position of the various chips has been specified, individual cavities or one large cavity having appropriate depth at the intended locations of differing chips, is prepared. This may be done by starting with a bare substrate having a uniform thickness and the desired size. Conventional, ultrasonic or laser milling may be used to form the cavities in which the various chips and other components will be positioned. For many systems where it is desired to place chips nearly edge-to-edge, a single large cavity is satisfactory. That large cavity may typically have a uniform depth where the semiconductor chips have a substantially uniform thickness. Where a particularly thick or a particularly thin component will be placed, the cavity bottom may be made respectively deeper or shallower to place the upper surface of the corresponding component in substantially the same plane as the upper surface of the rest of the components and the portion of the substrate which surrounds the cavity. The bottom of the cavity is then provided with a thermoplastic adhesive layer which may preferably be polyetherimide resin available under the trade name ULTEM.RTM. 6000 from the General Electric Company. The various components are then placed in their desired locations within the cavity, the entire structure is heated to about 300.degree. C. which is above the softening point of the ULTEM.RTM. polyetherimide (which is in the vicinity of 235.degree. C.) and then cooled to thermoplastically bond the individual components to the substrate. Thereafter, a polyimide film which may be Kapton.RTM. polyimide, available from E. I. du Pont de Nemours Company, which is .apprxeq.0.0005-0.003 inch (.apprxeq.12.5-75 microns) thick is pretreated to promote adhesion by reactive ion etching (RIE), the substrate and chips are then coated with ULTEM.RTM. 1000 polyetherimide resin or another thermoplastic and the Kapton film is laminated across the top of the chips, any other components and the substrate with the ULTEM.RTM. resin serving as a thermoplastic adhesive to hold the Kapton.RTM. in place. Thereafter, via holes are provided (preferably by laser drilling) in the Kapton.RTM. and ULTEM.RTM. layers in alignment with the contact pads on the electronic components to which it is desired to make contact. A metallization layer which is deposited over the Kapton.RTM. layer extends into the via holes and makes electrical contact to the contact pads disposed thereunder. This metallization layer may be patterned to form individual conductors during the process of depositing it or may be deposited as a continuous layer and then patterned using photoresist and etching. The photoresist is preferably exposed using a laser to provide an accurately aligned conductor pattern at the end of the process. Alternatively, exposure through a mask may be used.
Additional dielectric and metallization layers are provided as required in order to provide all of the desired electrical connections among the chips. Any misposition of the individual electronic components and their contact pads is compensated for by an adaptive laser lithography system which is the subject of some of the Patents and Applications which are listed hereinafter.
This high density interconnect structure provides many advantages. Included among these are the lightest weight and smallest volume packaging of such an electronic system presently available. A further, and possibly more significant advantage of this high density interconnect structure, is the short time required to design and fabricate a system using this high density interconnect structure. Prior art processes require the prepackaging of each semiconductor chip, the design of a multilayer circuit board to interconnect the various packaged chips, and so forth. Multilayer circuit boards are expensive and require substantial lead time for their fabrication. In contrast, the only thing which must be specially pre-fabricated for the HDI system is the substrate on which the individual semiconductor chips will be mounted. This substrate is a standard stock item, other than the requirement that the substrate have appropriate cavities therein for the placement of the semiconductor chips so that the interconnect surface of the various chips and the substrate will be in a single plane. In the HDI process, the required cavities may be formed in an already fired ceramic substrate by conventional or laser milling. This milling process is straightforward and fairly rapid with the result that once a desired configuration for the substrate has been established, a corresponding physical substrate can be made ready for the mounting of the semiconductor chips in as little as 1 day and typically 4 hours for small quantities as are suitable for research or prototype systems to confirm the design prior to quantity production.
The process of designing an interconnection pattern for interconnecting all of the chips and components of an electronic system on a single high density interconnect substrate normally takes somewhere between one week and five weeks. Once that interconnect structure has been defined, assembly of the system on the substrate may begin. First, the chips are mounted on the substrate and the overlay structure is built-up on top of the chips and substrate, one layer at a time. Typically, the entire process can be finished in one day and in the event of a high priority rush, could be completed in four hours. Consequently, this high density interconnect structure not only results in a substantially lighter weight and more compact package for an electronic system, but enables a prototype of the system to be fabricated and tested in a much shorter time than is required with other packaging techniques.
This high density interconnect structure, methods of fabricating it and tools for fabricating it are disclosed in U.S. Pat. No. 4,783,695, entitled "Multichip Integrated Circuit Packaging Configuration and Method" by C. W. Eichelberger, et al.; U.S. Pat. No. 4,835,704, entitled "Adaptive, Lithography System to Provide High Density Interconnect" by C. W. Eichelberger, et al.; U.S. Pat. No. 4,714,516, entitled "Method to Produce Via Holes in Polymer Dielectrics for Multiple Electronic Circuit Chip Packaging" by C. W. Eichelberger, et al.; U.S. Pat. No. 4,780,177, entitled "Excimer Laser Patterning of a Novel Resist" by R. J. Wojnarowski et al.; U.S. patent application Ser. No. 249,927, filed Sept. 27, 1989, entitled "Method and Apparatus for Removing Components Bonded to a Substrate" by R. J. Wojnarowski, et al.; U.S. Pat. No. 4,894,115, entitled "Laser Beam Scanning Method for Forming Via Holes in Polymer Materials" by C. W. Eichelberger, et al.; U.S. patent application Ser. No. 312,798, filed Feb. 21, 1989, entitled "High Density Interconnect Thermoplastic Die Attach Material and Solvent Die Attachment Processing" by R. J. Wojnarowski, et al.; U.S. Pat. No. 4,878,991, entitled "Simplified Method for Repair of High Density Interconnect Circuits" by C. W. Eichelberger, et al.; U.S. patent application Ser. No. 305,314, filed Feb. 3, 1989, entitled "Fabrication Process and Integrated Circuit Test Structure" by H. S. Cole, et al. and now abandoned; U.S. patent application Ser. No. 250,010, filed Sept. 27, 1988, entitled "High Density Interconnect With High Volumetric Efficiency" by C. W. Eichelberger, et al.; U.S. patent application Ser. No. 329,478, filed Mar. 28, 1989, entitled "Die Attachment Method for Use in High Density Interconnected Assemblies" by R. J. Wojnarowski, et al.; U.S. Pat. No. 4,960,613, entitled "Laser Interconnect Process" by H. S. Cole, et al.; U.S. Pat. No. 4,884,222 entitled "Method and Configuration for Testing Electronic Circuits and Integrated Circuit Chips Using a Removable Overlay Layer" by C. W. Eichelberger, et al.; U.S. patent application Ser. No. 233,965, filed Aug. 8, 1988, entitled "Direct Deposition of Metal Patterns for Use in Integrated Circuit Devices" by Y. S. Liu, et al. and now abandoned; U.S. Pat. No. 4,882,200, entitled "Method for Photopatterning Metallization Via UV Laser Ablation of the Activator" by Y. S. Liu, et al.; U.S. patent application Ser. No. 237,685, filed Aug. 25, 1988, entitled "Direct Writing of Refractory Metal Lines for Use in Integrated Circuit Devices" by Y. S. Liu, et al. and now abandoned; U.S. Pat. No. 4,933,042, entitled "Method and Apparatus for Packaging Integrated Circuit Chips Employing a Polymer Film Overlay Layer" by C. W. Eichelberger, et al.; U.S. Pat. No. 4,897,153, entitled "Method of Processing Siloxane-Polyimides for Electronic Packaging Applications" by H. S. Cole, et al.; U.S. patent application Ser. No. 289,944, filed Dec. 27, 1988, entitled "Selective Electrolytic Deposition on Conductive and Non-Conductive Substrates" by Y. S. Liu, et al.; U.S. patent application Ser. No. 312,536, filed Feb. 17, 1989, entitled "Method of Bonding a Thermoset Film to a Thermoplastic Material to Form a Bondable Laminate" by R. J. Wojnarowski and now abandoned; U.S. patent application Ser. No. 363,646, filed June 8, 1989, entitled "Integrated Circuit Packaging Configuration for Rapid Customized Design and Unique Test Capability" by C. W. Eichelberger, et al.; U.S. patent application Ser. No. 07/459,844, filed Jan. 2, 1990, entitled "Area-Selective Metallization Process" by H. S. Cole, et al.; U.S. patent application Ser. No. 07/457,023, filed Dec. 26, 1989, entitled "Locally Orientation Specific Routing System" by T. R. Haller, et al.; U.S. patent application Ser. No. 456,421, filed Dec. 26, 1989, entitled "Laser Ablatable Polymer Dielectrics and Methods" by H. S. Cole, et al.; U.S. patent application Ser. No. 454,546, filed Dec. 21, 1989, entitled "Hermetic High Density Interconnected Electronic System" by W. P. Kornrumpf, et al.; U.S. patent application Ser. No. 07/457,127, filed Dec. 26, 1989, entitled "Enhanced Fluorescence Polymers and Interconnect Structures Using Them" by H. S. Cole, et al.; U.S. patent application Ser. No. 454,545, filed Dec. 21, 1989, entitled "An Epoxy/Polyimide Copolymer Blend Dielectric and Layered Circuits Incorporating It" by C. W. Eichelberger, et al.; application Ser. No. 07//504,760, filed Apr. 5, 1990, entitled, "A Building Block Approach to Microwave Modules", by W. P. Kornrumpf et al.; application Ser. No. 07/504,821, filed Apr. 5, 1990, entitled, "HDI Microwave Circuit Assembly", by W. P. Kornrumpf, et al.; application Ser. No. 07/504,750 filed Apr. 5, 1990, entitled, "An Ultrasonic Array With a High Density of Electrical Connections", by L. S. Smith, et al.; application Ser. No. 07/504,803, filed Apr. 5, 1990, entitled, "Microwave Component Test Method and Apparatus", by W. P. Kornrumpf, et al.; application Ser. No. 07/504,753, filed Apr. 5, 1990, entitled, "A Compact High Density Interconnected Microwave System", by W. P. Kornrumpf; application Ser. No. 07/504,769, filed Apr. 5, 1990, entitled, "A Flexible High Density Interconnect Structure and Flexibly Interconnected System" by C. W. Eichelberger, et al.; application Ser. No. 07/504,751, filed Apr. 5, 1990, entitled, "Compact, Thermally Efficient Focal Plane Array and Testing and Repair Thereof", by W. P. Kornrumpf, et al.; application Ser. No. 07/504,749, filed Apr. 5, 1990, entitled, "High Density Interconnect Structure with Top Mounted Components", by R. J. Wojnarowski, et al.; application Ser. No. 07/504,770, filed Apr. 5, 1990, entitled, "A High Density Interconnect Structure Including a Chamber", by R. J. Wojnarowski, et al.; and application Ser. No. 07/504,748, filed Apr. 5, 1990, entitled, "Microwave Component Having Tailored Operating Characteristics and Method of Tailoring" by W. P. Kornrumpf, et al. Each of these Patents and Patent Applications is incorporated herein by reference.
Any additional dielectric layers which are required for isolation between the first metallization layer and any subsequent metallization layers or for prevention of short circuits due to contact with external conductors are formed by spinning on or spraying on a solvent solution of a desired thermoplastic dielectric material. The structure is then baked to drive off the solvent in order to leave a solvent-free dielectric layer. Thereafter, via holes are formed as needed and a patterned metallization layer is formed thereover which is disposed in ohmic contact with underlying metal in the via holes. If needed, further thermoplastic dielectric and metallization layers are formed thereover in a similar manner. Unfortunately, there are a limited number of dielectric materials which are suitable for use as these upper dielectric layers because of all of the material properties they must exhibit. Not only must the dielectric material be available as a spin-on or spray-on solution, it must also provide good adhesion to the underlying dielectric and metallization and to the material of any overlying metallization or dielectric layer which may subsequently be formed thereon.
By a thermoplastic polymer material, we mean that upon multiple cycles of heating and cooling, the melting point of the polymer material remains substantially unchanged. That is, no substantial cross-linking of the material takes place during the heating, melting and resolidifying process. Such polymers are suitable as adhesive layers for bonding higher temperature materials, including polymers to substrates and may also themselves be used as layers to be bonded to substrates through the use of lower temperature adhesive layers. By a thermoset polymer material, we mean a polymer material in which cross-linking takes place the first time it is heated or during its preparation process, such that the material either no longer melts or melts at a much higher temperature than it did prior to its initial heating or its formation, as the case may be.
Microwave systems are often composed of monolithic microwave integrated circuits (MMICs), other active microwave devices such as GaAs transistors, passive microwave components and other non-microwave components such as logic and control structures.
A monolithic microwave integrated circuit or MMIC is an integrated circuit which is designed to operate at microwave frequencies. MMICs are normally fabricated in GaAs because of the much higher potential operating frequency which GaAs provides as compared to silicon. A typical MMIC may include one or more amplifiers, some passive components and one or more feedback loops which provide feedback from the output of an amplifier or circuit to establish a desired transfer function for that circuit.
It is known in the art to fabricate microwave systems from a variety of such components by providing a ceramic substrate having microstrip RF circuitry, DC supply lines (conductors), logic lines, control lines and contact pads fabricated thereon and by attaching devices and components such as MMICs, GaAs transistors, other microwave and supporting components to the substrate and connecting them to the circuitry on the substrate using wire bonds or tab interconnections.
Such fabrication techniques have a number of disadvantages. Thin and thick film methods of fabricating circuitry on ceramic substrates have tolerance limitations which prevent such structures from being produced with microwave characteristics which are repeatable with close tolerances. Consequently, there is substrate-to-substrate variation in the microwave characteristics of such nominally identical substrates. Further, the active microwave components such as MMICs and GaAs transistors themselves have fabrication tolerances which result in variations in operating characteristics from device to device. Further, in such structures, impedance discontinuities and mismatches are normal at the edges of MMICs and GaAs transistors. These impedance discontinuities vary with the actual placement of the chips on the surface of or in cavities in the ceramic substrate. This is because slight changes in the positioning of such devices change both the lengths of the gaps between the device and the substrate and the alignment of the device structure with the substrate structure. Further, these physical assembly tolerances result in variable interconnect bond lengths, resulting in variable inductances and consequently, varied circuit performance. These impedance mismatches also vary with actual component and substrate impedance values. Further, these and other fabrication-tolerance-induced differences in impedances result in reflections and other undesirable operational effects which degrade system operating characteristics. The cumulative effect of these differences is a wide range of system operating characteristics. Consequently, assembly of a microwave system from such components is a relatively low yield process in which many of the resulting systems do not meet specifications. A significant contributor to this low yield is the fact that many active microwave components cannot readily be accurately tested over their full expected operating frequency and power ranges in a non-destructive manner because of the difficulty of coupling such components to a test system. Consequently, many components which pass preassembly testing do not in reality meet specifications.
As the desired operating frequency of such microwave systems has increased from the neighborhood of 2 GHz to still higher frequencies in the range from 8 GHz to 16 GHz, or more, the problem of thin film and thick film fabrication tolerances and component testing have become ever more vexing.
Many MMICs and other active microwave devices include delicate structures which can easily be damaged or destroyed. These include conductors which are spaced from the surface of the GaAs by an air gap--a structure which is known as an "air bridge". Air bridges are used in these MMICs in order to provide the MMIC with particular desired operational characteristics. These delicate structures severely limit the assembly techniques which can be used to connect these devices into microwave systems. Further, such components are quite sensitive to the placement near their surfaces of conductors or dielectric materials having dielectric constants of more than one, especially in the vicinity of inductors, air bridges and field effect device gate regions.
The interconnection of microwave structures presents many problems, considerations and challenges not faced in the interconnection of digital systems. Use of microwave frequencies requires consideration of wave characteristics, transmission line effects, material properties at microwave frequencies, the presence of exposed delicate structures on MMICs and other components and system and component characteristics which do not exist at the lower operating frequencies of such digital systems. These considerations include the question of whether the dielectric materials are suitable for use at microwave frequencies, since materials which are good dielectrics at lower frequencies can be quite lossy or even conductive at microwave frequencies. Further, even if the dielectric is not lossy at microwave frequencies, its dielectric constant itself may be high enough to unacceptably modify the operating characteristics of MMICs, GaAs transistors and other microwave components or structures which might be interconnected using a high density interconnect structure. Since the first dielectric layer of this high density interconnect structure is applied by a lamination process involving the application of substantial pressure to the polyimide film, there is a substantial concern that air bridges and other delicate structures in microwave components may be damaged, destroyed or modified either by the lamination pressure causing them to collapse or by the infiltration of the thermoplastic adhesive into the air gap under the conductor, thereby modifying the dielectric properties of that gap, or even the mere presence of the dielectric unacceptably modifying the operating characteristics of some of the components.
Many of these devices are so sensitive to the presence of overlying dielectric layers that their manufactures do not even deposit glass passivation layers on the upper surface of those chips because even that thin dielectric layer would adversely affect the operating characteristics of the components. The use of glass passivating coatings on semiconductor chips is essentially universal in the silicon semiconductor art to prevent environmentally induced deterioration of the devices. Consequently, in the microwave art disposing any additional dielectric material on an active device or component is looked upon as a sure way to degrade performance and is assiduously avoided.
The Kapton polyimide typically used in this high density interconnect structure has the initial laminated dielectric layer as a dielectric constant of substantially 3.5. The ULTEM 1000 resin normally used as the thermoplastic adhesive for this Kapton layer has a dielectric constant of substantially 3.5. SPI siloxane polyimide which is typically used as the spin-on dielectric layer, has a dielectric constant of 2.7-2.9.
Use of a lower dielectric constant material as the dielectric layers of such a high density interconnect structure would reduce the capacitive loading and coupling experienced by the integrated circuit chips which the system interconnects and would also reduce the RC time constant of the conductive runs within the high density interconnect structure.
The polyimide and polyetherimide materials used in fabrication of the high density interconnect structure discussed above have excellent adhesion properties for semiconductor chips and metallization runs. These materials are also laser ablatable at 351 nm as has been discussed above.
The best known low dielectric constant polymer is the fluoropolymer, Teflon, available from DuPont Chemicals. The low dielectric constant of Teflon would make it attractive for use as a dielectric layer in a high density interconnect structure intended for operation at high frequencies. Unfortunately, Teflon exhibits a number of properties which mitigate against its use as the dielectric layer in a high density interconnect structure. First, Teflon is substantially transparent to 351 nm light and available Teflon film are opaque. Consequently, Teflon cannot be laser drilled at 351 nm. Second, an opaque film is unusable in a high density interconnect structure because of the need to be able to see through the dielectric layer for alignment and other purposes. Third, the adhesion properties of metal layers disposed on Teflon are uncertain, but the known non-stick properties of Teflon make such adhesion doubtful.
DuPont has recently made Teflon available as an amorphous fluoropolymer which can be deposited from a solvent solution. The resulting film is substantially transparent, which overcomes the drawback of previously available films of not being able to see the underlying structure through the film. However, this does not solve the other drawbacks of the Teflon films.
The problem of laser non-ablatability of the amorphous Teflon at 351 nm can be overcome by including an appropriate dye in the solvent Teflon solution as taught in application Ser. No. 456,421, entitled "Laser Ablatable Polymer Dielectrics and Methods" which is listed above or by using a different laser frequency. However, the problem of adhesion would still not be solved.
Consequently, there is a need for a low dielectric constant polymer dielectric material for use in high density interconnect structures which provides good adhesion, both to an underlying portion of the structure and to an overlying portion of the structure.